Method for producing hexagonal boron nitride single crystal and hexagonal boron nitride single crystal

ABSTRACT

An object of the present invention is to provide a novel method of growing hexagonal boron nitride single crystal. It is found that hexagonal boron nitride single crystal is grown in calcium nitride flux by heating, or heating and then slowly cooling, boron nitride and calcium series material in atmosphere containing nitrogen. Bulk of hexagonal boron nitride single crystal can thereby successfully be grown.

FIELD OF THE INVENTION

The present invention relates to hexagonal boron nitride single crystals and a method of producing the same.

BACKGROUND ARTS

Hexagonal boron nitride (h-BN) takes a stable phase having planar graphite structure of SP2 bonds. Hexagonal boron nitride single crystal has superior electrical insulation, a low dielectric constant, heat resistance, chemical stability and lubricating property. It has thus been expected as materials for electronics and machineries. In particular, boron nitride has a large band gap of about 6 eV and thus excellent as a material for a substrate for luminous devices (LED, LD) of ultraviolet wavelength region. It has been demanded a development of production technique of single crystal wafer.

It is, however, very difficult to grow bulk single crystal of hexagonal boron nitride. This is because hexagonal boron nitride hardly vaporizes until a high temperature and does not easily dissolve, and it has not been found a flux appropriate for growing hexagonal boron nitride.

It was disclosed a production technique for growing hexagonal boron nitride single crystal by flux process (solution process) in Japanese Patent publication No. 2001-72499A.

It has been further known that aluminum nitride single crystal is obtained by heating aluminum nitride and calcium nitride in sealed state at, for example, 1550° C. or 1610° C. (Mat. Res. Bull. Vol. 9 (1974) pages 331 to 336).

DISCLOSURE OF THE INVENTION

It has not however, known a method of growing hexagonal boron nitride single crystal of a high quality (low defect density) and of a large size yet. It has thus been demanded a novel growing method.

An object of the present invention is to provide a novel method of growing hexagonal boron nitride single crystal.

The present invention provides a method of producing a hexagonal boron nitride single crystal, the method comprising the step of:

heating a material comprising at least calcium atom and boron nitride to grow a hexagonal boron nitride single crystal.

The present inventors have studied a method of possibly growing hexagonal boron nitride single crystal of high quality and a large size. They have thus found that bulk of hexagonal boron nitride single crystal can be precipitated and grown in solution, by heating boron nitride and a material comprising at least calcium atom (calcium series material) in an atmosphere containing at least nitrogen atom. The present invention is thus made.

According to Mat. Res. Bull. Vol. 9 (1974) pages 331 to 336, aluminum nitride material and calcium nitride material are mixed and then heated to successfully precipitate single crystal of aluminum nitride. Such calcium containing flux is, however, extremely high in the reactivity so that it is susceptible to the reaction with crucible material to melt the crucible. It is further described that the sealed crucible may be broken as the inner pressure is increased. Due to these problems, it is considered that the growth of boron nitride single crystal using calcium-series flux has not been studied until the present invention is made.

BEST MODES FOR CARRYING OUT THE INVENTION

Although the calcium series material is not limited as it contains calcium atom, calcium nitride or calcium metal are most preferred.

The shapes of the boron nitride and calcium series materials are not particularly limited, and may be bulk or powder. Further, although the boron nitride and calcium series materials may preferably be of a high purity, it may contain a predetermined amount of a dopant depending on the application.

According to the present invention, the boron nitride and calcium series materials are weighed, charged in a crucible and then heated under pressure in nitrogen-containing atmosphere to grow hexagonal boron nitride single crystal. The material of the crucible is not particularly limited as far as it is not reactive with the calcium containing flux at the growing temperature. For example, it is found that titanium nitride and zirconium nitride can appropriately be used.

Although the partial pressure of nitrogen during the growth is not particularly limited, it may preferably be not lower than atmospheric pressure for avoiding the evaporation of the materials. Although the upper limit is not particularly limited, it may be 10 atoms or lower on the practical viewpoint.

The atmosphere may be composed of nitrogen only, or may contain a gas other than nitrogen. Such gas other than nitrogen includes argon and helium. In the case that the nitrogen-containing atmosphere contains the gas other than nitrogen, the total pressure of the atmosphere is not particularly limited.

Further, although the temperature during the growth is appropriately selected, it may be 800 to 1800° C., for example. On the viewpoint of assisting the reaction, the growing temperature may preferably be 1200° C. or higher. As the temperature exceeds 1600° C., the vapor pressure of Ca is considerably elevated. Thus, the growing temperature may more preferably be 1600° C. or lower.

Further, although the cooling rate after the heating is not limited, slow cooling is preferred on the viewpoint of precipitating a larger crystal. Although the degree of the slow cooling is not particularly limited, fine crystals may become predominant as the cooling rate is large. Thus, the cooling rate may preferably be 10° C./hour or smaller and more preferably be 5° C./hour or smaller, on the viewpoint of precipitating target single crystal. On the viewpoint of improving the crystal quality, the cooling rate may most preferably be 1° C./hour or smaller. Further, the cooling rate may preferably be 1° C./day or larger on the viewpoint of improving the growth rate.

The ratio of the boron nitride and the calcium series material may preferably be that BN: Ca=1:3 to 3:1 as the converted molar ratio calculated as boron nitride and calcium, respectively.

Further, in the case that the calcium series material is calcium nitride, the ratio of boron nitride and calcium nitride may preferably be that BN: Ca₃N₂=1:9 to 5:5 as the molar ratio.

Further, to the materials, it may be added alkali metal such as sodium or lithium or an alkaline earth metal such as barium, strontium or magnesium. Further, a metal of a low melting point such as tin, indium or zinc may be added. By adding the materials, the solubility of boron nitride can be improved or the melting point of the materials can be lowered so that the growing temperature can be lowered. On the viewpoint of improving the solubility, an alkali metal or an alkaline earth metal is preferred, and sodium is most preferred. The melting point of the added metal may preferably be lower on the viewpoint of the growing temperature.

Besides, the metal of a low melting point means a metal having a melting point of 500° C. or lower.

In the case that sodium is added, an HIP system may preferably be used for preventing the evaporation of the materials.

EXAMPLES

Although preferred examples of the present invention will be described below, the present invention is not to be limited to the following examples and various modifications can be made.

Example 1

Boron nitride powder (purity of 99.99 percent) and calcium nitride powder (purity of 99.99 percent) were weighed in a glove box so that the molar ratio (BN: Ca₃N₂) is 10:90. These materials were charged into a crucible of zirconium nitride. The crucible was sealed in a container of stainless steel. Nitrogen gas was filled into the container, and then heated to 1550° C. and pressurized to 10 atms. After it was held at 1550° C. and 10 atms for 1 hour, the temperature was slowly lowered for 50 hours at a rate of 2° C./ hour.

As a result, it was successfully obtained a plate-like transparent crystal having a size of 3 mm and a thickness of about 0.5 mm. It was confirmed by X-ray diffraction analysis, that this crystal was hexagonal boron nitride single crystal.

Example 2

Boron nitride powder (purity of 99.99 percent) and calcium metal powder (purity of 99.99 percent) were weighed in a glove box so that the molar ratio (BN: Ca) is 1:3. These materials were charged into a crucible of titanium nitride. The crucible was sealed in a container of stainless steel.

Nitrogen gas was filled into the container, and then heated to 1550° C. and pressurized to 10 atms. After it was held at 1550° C. and 10 atms for 1 hour, the temperature was slowly lowered for 50 hours at a rate of 2° C./ hour and then naturally lowered to room temperature.

As a result, it was successfully obtained a plate-like hexagonal boron nitride single crystal having a size of 2 mm and a thickness of about 0.5 mm, confirmed by X-ray diffraction analysis.

Example 3

Boron nitride powder (purity of 99.99 percent), calcium metal (purity of 99.99 percent) and sodium metal were weighed in a glove box so that the molar ratio (BN:Ca:Na) is 6:2:1. These materials were charged into a crucible of titanium nitride. The crucible was sealed in a container of stainless steel. Nitrogen-argon gas mixture was filled in the container using an HIP system, which was then heated to 1400° C. and pressurized to 1000 atms. The partial pressure of nitrogen was made 10 atms. After it was held at 1400° C. for 50 hours, the temperature was naturally lowered to room temperature.

As a result, it was successfully obtained a plate-like transparent crystal having a size of 3 mm and a thickness of about 0.5 mm. it was confirmed, by X-ray diffraction analysis, that this crystal was hexagonal boron nitride single crystal.

Example 4

Boron nitride powder (purity of 99.99 percent), calcium metal (purity of 99.99 percent) and tin metal were weighed in a glove box so that the molar ratio (BN:Ca:Sn) is 6:2:1. These materials were charged into a crucible of titanium nitride. The crucible was sealed in a container of stainless steel. Nitrogen gas was filled into the container, and then heated to 1400° C. and pressurized to 10 atms. After it was held at 1400° C. for 1 hour, the temperature was lowered for 50 hours at a cooling rate of 2° C./hour and then naturally cooled to room temperature.

As a result, it was successfully obtained a plate-like transparent crystal having a size of about 2 to 3 mm and a thickness of about 0.5 mm. it was confirmed, by X-ray diffraction analysis, that this crystal was hexagonal boron nitride single crystal. 

1. A method of producing a hexagonal boron nitride single crystal, the method comprising the step of: heating a material comprising at least calcium atom and boron nitride in an atmosphere comprising at least nitrogen to grow a hexagonal boron nitride single crystal.
 2. The method of claim 1, further comprising the step of slow cooling after said heating step.
 3. The method of claim 1, wherein said material comprises calcium nitride.
 4. The method of claim 1, wherein said material comprises calcium metal.
 5. The method of claim 1, wherein one or more metal selected from the group consisting of an alkali metal and an alkaline earth metal is mixed with said material and said boron nitride before said heating.
 6. The method of claim 1, wherein one or more kind(s) of a metal having a low melting point is mixed with said material and boron nitride before said heating.
 7. The method of claim 1, wherein said boron nitride single crystal is grown in a crucible comprising zirconium nitride.
 8. The method of claim 1, wherein said boron nitride single crystal is grown in a crucible comprising titanium nitride.
 9. A hexagonal boron nitride single crystal grown by the method of claim
 1. 